1. Field of the Invention
The present invention relates to a glass substrate (of various shapes, including plate-like and not plate-like, for example, rod-like) having a plurality of fine hole(s) therein, applicable to various technical fields, including providing guide holes for use in the mounting of fibers in the field of optical communication, or providing holes for use in ink injection in the field of printers.
2. Description of Related Art
A substrate having a plurality of fine or minute holes therein has already been put to practical use comprising various materials, such as polyimide, or Teflon (trade mark of polytetrafluoroethylene), and such substrates are now used in several practical applications, providing contact holes for making electrical connection between layers in a multilayer printed circuit board, providing ejecting openings for inkjet printers, and providing holes for insertion of fibers for a fiber array, among others.
The holes used for the applications mentioned above have a diameter from several tens of .mu.m to several hundreds of .mu.m, and the holes are formed using a laser beam in most cases. For the laser beam, there is used infrared laser beam produced by, for example, a CO.sub.2 laser, or a laser covering from the near-infrared region to the visible region, or further covering the ultraviolet region, by use of a Nd-YAG laser or by combining a Nd-YAG laser and a wavelength converter. Also, there is used a laser machining device using an excimer laser employing KrF (wavelength: 248 nm).
In addition, among various kinds of glass, a silicate glass containing mainly SiO.sub.2 shows superior chemical stability and is also stable at high-temperature. Therefore, this glass can be applied to a large number of uses with the fine holes being formed therein by microscopic machining.
For performing the machining of such fine holes on the above-mentioned silicate glass, generally, they are machined by use of the laser beam mentioned above, or by a wet etching process using an etchant, such as fluoric acid, or by using a drill.
However, in the wet etching treatment, there are problems in that cylindrical shape holes cannot be obtained. Also, there are problems in the care and treatment of the etchant, and when machining the holes by using a drill, there is a limit to forming fine holes of around a diameter of 0.5 mm, as taught by Japanese Patent Laid-Open No. Sho 62-128794 (1987). Therefore, it is difficult to form a very fine or minute hole beyond that limit.
Additionally, trials have been performed by applying a laser beam to the microscopic machining of glass, as disclosed in Japanese Laid-Open Patent No. Sho 62-128794 (1987). However, the glass is inherently a fragile material, and cracks are easily caused therein when it is machined. In particular, even when using a KrF excimer laser in the ultraviolet region (wavelength 248 nm), cracks occur around irradiation marks or traces, and further the hole cannot be finished with a smooth interior wall. Therefore, a substrate having fine holes of sufficient quality cannot be formed.
Also, a technology was proposed which is disclosed in Japanese Laid open Patent No. Sho 54-28590 (1979). In this prior art, the glass is heated at 300-700.degree. C. prior to being machined by irradiation of a laser beam thereon, so that it can withstand the thermal shock caused during the machining.
However, as mentioned in the above, if performing the microscopic machining on the glass with a laser beam under such conditions so that the thermal stress is relieved, since thermal shrinking occurs, it is impossible to maintain accuracy in machining in the order from micrometers to sub-micrometers. Further, a complicated device is necessitated for irradiating the laser beam on the glass under the condition that it is heated. Therefore, this process is not realistic from the viewpoint of efficient manufacturing.
Consequently, it is important to obtain a method of forming a glass substrate with fine holes therein, each of which has a smooth interior surface without cracks around the opening thereof, and that can be obtained without using means such as the heating mentioned above.
As a result, the inventors tried to irradiate a laser beam on common photosensitive glass which contains Ag ions in a uniform concentration therein. Explaining the process thereof by referring to FIGS. 1(a) to (d), as shown in FIG. 1(a), the laser beam was irradiated upon the glass substrate and entered into the inside thereof, and as shown in FIG. 1(b), the laser beam energy resolves the Ag ion residing inside thereof, thereby producing colloid (amicron of Ag). In forming the colloid, the coefficient of absorption of the laser beam was increased greatly as shown in FIG. 1(c), and then abrasion occurred from the inside thereof. Finally, as shown in FIG. 1(d), a concave portion was formed which is similar to cracking or fragmenting. Therefore, the process for forming the holes was stopped.